A power supply device that uses a generator driven by an engine as a power supply is disclosed in, for example, Japanese Patent Application Laid-Open Publication No. 2000-217398 and Japanese Patent Application Laid-Open Publication No. 5-18285, which includes: a power supply unit that uses the generator as the power supply to output a DC voltage; an inverter device that converts the DC output of the power supply unit into an AC output at a fixed frequency; and an engine rotational speed control unit that controls a rotational speed of the engine so as to be kept at a target rotational speed suitable for a load applied to the inverter device. The target rotational speed is a rotational speed of the engine required for generating a necessary output for driving the load from the inverter device.
When an AC generator is used as a generator, a power supply unit is comprised of the AC generator and a converter that converts an AC output to a DC output.
The inverter device is comprised of a known bridge type inverter circuit, each side of a bridge including switch elements and feedback diodes connected anti-parallel to the switch elements, and an inverter control unit that controls on/off of the switch elements that constitute the inverter circuit at a predetermined timing so as to output an AC voltage at a predetermined frequency from the inverter circuit.
The engine rotational speed control unit provided in the conventional inverter controlled generator set includes, for example, a target rotational speed setting unit that sets a target rotational speed of the engine according to the size of the load of the inverter, a rotational speed detection unit that detects a rotational speed of the engine, a deviation detection unit that detects a deviation between the detected rotational speed and the arithmetically operated target rotational speed, an operation amount arithmetical operation unit that arithmetically operates an operation amount of an intake air amount adjusting unit (for example, a throttle valve) of the engine required for bringing the deviation to zero by performing a proportional arithmetical operation (a P arithmetical operation), a proportional plus integral plus derivative arithmetical operation (a PID arithmetical operation), or a proportional plus integral arithmetical operation (a PI arithmetical operation) of the detected deviation, and an operation unit that operates the intake air amount adjusting unit of the engine by the amount of operation arithmetically operated by the operation amount arithmetical operation unit, and the engine rotational speed control unit uses feedback to control the rotational speed of the engine so as to bring the deviation to zero between the rotational speed of the engine and the target rotational speed.
The inverter controlled generator set, including the engine rotational speed control unit that sets the target rotational speed suitable for the size of the load and use feedback to control the rotational speed of the engine so as to bring the deviation to zero between the rotational speed of the engine and the target rotational speed as described above, has the following problem when intermittently driving a load requiring a large starting current, for example, an electric power tool or the like.
For an inverter controlled generator set that drives a load requiring a large starting current such as an electric power tool, when an operator drives the load, an engine rotational speed control unit sharply increases a rotational speed of an engine to about an upper limit value to allow an inverter to pass a large starting current through the load. When the operator then stops the operation, the load is eliminated, and thus the engine rotational speed control unit sharply reduces the rotational speed of the engine to a lower limit value (usually an idling rotational speed). When the rotational speed of the engine is sharply increased and reduced, the engine makes a loud operation sound. The load intermittently driven such as the electric power tool is repeatedly started and stopped in a relatively short cycle, and thus if an engine driven inverter controlled generator set is used for driving such a load, a significant variation of the rotational speed of the engine frequently causes a loud operation sound. The operation sound of the engine and an operation sound made by the load itself produce a loud noise, which annoys the operator or people around.